The invention relates to a method and a device for treating sludge, especially sewage sludge, whereby dry sludge is mixed into mechanically predewatered thick sludge, the mixture is dried to form dry sludge, and the dry sludge is cooled before being transported away.
Such a sludge drying serves to condition the sludge primarily originating in water (sewage) treatment plants such that later on, optionally after intermediate storage, it can be disposed of, for example, by combustion in a refuse incinerator or as fertilizer in agriculture.
The sewage sludge, in general, contains an amount of dry substance of approximately 2 to 5 mass %. The mechanical predewatering process increases the dry substance contents to approximately 20 to 30 mass %. This thick sludge cannot be subjected directly to the actual drying process. Indeed, the drying process requires a dry substance contents of approximately 50 to 80 mass %. Therefore, the thick sludge is mixed with already dried sludge. During drying the dry substance contents of the mixture is increased to approximately 95 mass %. During this process the temperature of the dry sludge increases to approximately 120.degree. C. In general, this temperature must be lowered before transporting the dry sludge to approximately 50.degree. to 70.degree. C. because otherwise an unacceptably high risk of self ignition is present.
European patent application 0 458 221 describes a method and a device of the aforementioned kind. Here, the temperature of the dryer is controlled by adjusting the mixture ratios between the thick sludge and the dry sludge. For this purpose, the gas temperatures of the recirculating gas is determined and fed into the control device. The latter controls the metering device and thus the amount of dry sludge to be admixed.
It has been found that the conventional sludge treatment needs improvement with respect to economic efficiency.
Accordingly, it is an object of the invention to provide a possibility for sludge treatment with increased efficiency.